1. Field of the Invention
The present invention relates to a method for manufacturing foamed plastics and particularly to a method in which a plastic material is molten and plasticized by induction heating and a foaming agent contained in the plastic material is dissolved to form a plastic product of a predetermined shape.
2. Description of the Related Art
A molded product made of foamed plastics, such as an elongated strip of a decorative and protective molding which can be attached to a side panel or a door panel of an automobile is light in weight and has a large shock-absorbing effect, in comparison with a solid molding.
This sort of foamed plastic molding can be made in a process wherein a piece of raw material which consists of a resin material with a foaming agent mixed therein is introduced in molding dies which are provided with high frequency electrodes, and is heated by an induction heating process to melt and plasticize it, so that the foaming agent contained in the resin material is dissolved, resulting in the production of a plastic molding having a predetermined shape.
The plastic molding thus obtained has the largest cell construction at its interior center core portion, since the interior foaming agent dissolves and foams when heated by the application of high frequency voltage. On the other hand, the outer surface of the plastic molding forms a hard and dense shell. Accordingly, the plastic molding as mentioned above can be advantageously used as a decorative and protective molding for an automobile.
However, in the plastic molding, if the foaming agent has a large extent of foaming or a high expansion coefficient, the interior foams cause the outer shell to expand, thus resulting in the appearance of a rough skin which is not desireable in a decorative molding for an automobile.
It will be noted that the foaming agent used to make decorative and protective molding for an automobile usually has a high expansion coefficient to provide an inexpensive molded product having a high shock-absorbing or damping effect. The rough skin results in an unacceptably bad appearance for the molding product and cancels the peculiar advantages of the plastic molding as mentioned above and, thereby, decreases the practicabilty of the plastic molding.